The present invention relates to an electric power steering apparatus.
Conventionally, a power steering apparatus for a vehicle includes an electric power steering apparatus (EPS) provided with a motor as a driving source. The EPS has a feature that a flexibility of the layout is high, and an energy consumption is small, in comparison with a hydraulic power steering apparatus. Accordingly, in recent years, the introduction of the EPS has been considered for many kinds of vehicles ranging from compact vehicles to large-sized vehicles.
In the EPS mentioned above, a vibration generated in a steering system is one of factors that deteriorate the steering feel. In other words, if a driver feels vibration of the steering system or an abnormal noise caused by the vibration, the steering feel is greatly deteriorated. Accordingly, in the conventional EPS, for example, as disclosed in Japanese Laid-Open Patent Publication Nos. 2006-27537 and 2006-335228, various countermeasures are devised on a structure and on a control, for suppressing vibration of the steering system.
However, vibration generated in the steering system is not necessarily produced from the motor serving as the driving source. In other words, in the case that a stress is applied to steerable wheels such as traveling on a rough road surface, the vibration remains in the steering system until the stress is attenuated. The vibration is transmitted to the steering wheel, and can deteriorate the steering feel.
Japanese Laid-Open Patent Publication No. 2006-335228 discloses a structure which extracts a vibration frequency component from a motor rotation angle and a current value corresponding to a control output of the motor, and applies a vibration suppression control amount for canceling the vibration frequency component. In accordance with this configuration, it is possible to suppress the vibration generated in the steering system to some extent, in the EPS in which the motor and the steering system are coupled. However, since the vibration component of the steering system extracted from the control output of the motor is an indirect one, a phase shift is generated in a compensation control based on the vibration component. Therefore, there is a limit in an effect of the compensation control, and an effective countermeasure is desired for effectively suppressing the vibration of the steering system.